1. Field of the Invention
The present invention relates to an image sensor capable of being employed in a facsimile system, copier, scanner, or the like for reading light reflected from a read surface, and to an image reading apparatus using the image sensor.
2. Description of Related Art
In the past, a close-contact type image sensor using a sensor array to form a read image on a one-to-one reduced scale and thus read an image from a read original with the size of the image unchanged has been used as an original reading device to be employed in a facsimile system, copier, scanner, or the like.
A known method is, as described in Japanese Patent Publication No. Hei 8-34531, such that a rod lens array and a line light source are fixed directly to a housing by performing affixation or the like, and then a contact glass is fixed to the housing. However, the work is complex. Besides, since the rod lens array and the line light source are affixed mutually independently, a positional deviation is likely to occur. There is the fear of a failure to read an image properly.
FIG. 7 is a sectional view of a known example of a close-contact type image sensor in which the above drawbacks are taken into account. The close-contact type image sensor has a housing 1 serving as a supporting means in which a sensor substrate 10 on which a sensor part 4 composed of a plurality of sensor chips for defining the locations of a plurality of pixels and for carrying out photoelectric conversion, and a protective film for protecting the plurality of sensor chips are mounted, and an optical member 3 including light sources 13 attached to an end face thereof for irradiating light to a read original, a rod lens array 2 that is a lens for forming an image read from the read original as the pixels on the surface of the sensor part 4, and a contact glass 5 serving as an original reading plane, are mounted. Attached to the sensor substrate 10 are an image processing circuit 11 for processing an image signal read by the sensor part 4 composed of the sensor chips and a connection part 12 for externally outputting an output signal of the close-contact type image sensor. The rod lens array 2 and the optical member 3 are pressed to the housing 1 by the contact glass 5. The contact glass 5 is fixed to the housing 1 by performing affixation or the like.
However, in this known example, since the contact glass 5 determines the focal length of the rod lens array 2, there is no range of choice in determining the thickness of the contact glass 5. Therefore, a special glass having a specific thickness must be custom-made.
In recent years, the number of scenes in which color images are handled has increased with the prevalence of color printers and color displays. There is an increasing demand for reading of color originals rather than conventional monochrome originals.
When a close-contact type image sensor is used to read a color original, light emitted from a light source and falling on the original reading plane must be multi-color light so that the color original can be read. In general, trichromatic light of red, green, and blue is used as light incident on the original reading plane. Three light sources mutually separated to be associated with the three colors or a single light source formed with a recently-developed white LED for emitting white light having the three colors mixed therein is included in the close-contact type image sensor.
Moreover, studies have been made of a close-contact type image sensor for reading a color original, in which light reflected from the original reading plane and segmented into colors is passed through a rod lens array that is a lens for forming an image as pixels on the surfaces of sensor chips, guided to the plurality of sensor chips arranged to define the locations of pixels and designed to carry out photoelectric conversion, and then photoelectrically converted color by color.
An information processing apparatus using the close-contact type image sensor, for example, a scanner, may adopt either a flat-bed system or a sheet-through system.
The flat-bed system is such that a book or any other original is placed on a top glass with an information side thereof facing the top glass, and a close-contact image sensor opposed to the information side is moved to read image information from the original.
By contrast, the sheet-through system is such that mutually-independent original sheets are moved one by one while brought into close contact with a close-contact type image sensor.
In the known example, for coping with both the systems, the contact glass 5 must be replaced with another.
The prior art will be described in conjunction with FIGS. 5 and 6. FIG. 5 is a sectional view showing the structure of a close-contact type image sensor of the sheet-through system. FIG. 6 is a sectional view showing the structure of a close-contact type image sensor of the flat-bed system. The structure in accordance with the prior art will be described. That is to say, an optical member 3 serving as a light source unit including light sources 13 and a rod lens array 2 are pressed from above in the drawing by means of a cover glass 6 and located at desired positions in a housing 1. The cover glass and a housing 1 are secured by performing affixation or the like. As a result, the optical member 3 and the rod lens array 2 are fixed to the housing 1.
A sensor part 4 mounted on a sensor substrate 10 is formed by placing a sensor array on the sensor substrate 10, electrically connected to an image processing circuit 11, resistor, and capacitor, and also electrically connected to a connection part 12 enabling connections with external circuits.
In FIG. 5, the contact glass 5 which is arranged to be brought into contact with an original P is fixed to the cover glass 6. The reasons therefor will be described below.
(i) The thickness of a glass matching the focal length of the rod lens array 2 can be set to a selected value more easily by forming the glass with two glasses of the contact glass 5 and cover glass 6 than by forming the glass with a single glass. Consequently, at least the rod lens array 2 and the optical member 3 can be fixed at the same time.
(ii) For the one kind of housing 1, either the flat-bed system or the sheet-through system can be selected depending on the presence or absence of the contact glass 5.
In FIG. 6, the contact glass 5 is removed, and a top glass 9 is disposed in such a way as to ensure an optically equivalent structure. The optical member 3 serving as a light source unit uses a light guide to diffuse light rays emitted from the light sources (LEDs) 13 located at both ends in a longitudinal direction of the optical member 3, and to guide the diffused light to an original P. The original P is then illuminated with the light. Light reflected from the original P passes through the rod lens array 2, forms an image on the sensor part 4, and is then converted into an electrical signal. For reading a color image, LEDs for emitting light rays of three colors of red, green, and blue are included and lit.
However, as far as the foregoing close-contact type image sensor of the prior art is concerned, even when the surfaces of the contact glass 5 and the cover glass 6 shown in FIG. 5 which are in contact with each other are flattened to as great an extent as mechanically and economically possible, a gap of several um deep would exist partly on a contact plane 20 between the contact glass 5 and the cover glass 6. Therefore, interference fringes are produced by interference of light. Accordingly, an output wave undergoes the adverse effect of interference fringes and exhibits inhomogeneities associated with the interference fringes.
Moreover, the depth of the gap varies depending on the pressure given by a roller used to transport an original or depending on ambient temperature or humidity. The output wave varies accordingly. This poses a problem that since correction cannot be achieved properly, a read error occurs.
For reading especially a color image, when light rays with long wavelengths interfere with one another, interference fringes are likely to occur. The wavelengths of red, green, and blue light rays become different because of the interference fringes. Consequently, the problem that a color deviation occurs in read data becomes outstanding. The adverse effect of the interference fringes becomes more serious than that occurring when a monochrome image is read.